Active noise reduction (ANR) earplugs have been identified as viable means for suppressing the sound pressure levels inside users' ear canals. The primary advantage of the ANR earplug is its ability to suppress noise inside a smaller occluded space than the corresponding occluded space in prior ANR earmuff designs. This reduction in the volume of the occluded space causes the corresponding reduction in dimension of the acoustic dynamic system to be controlled, thereby allowing control of a broader frequency bandwidth. A significant disadvantage associated with prior ANR earplugs is that all earplug constructions offered thus far cannot guarantee that the electronic control attenuation at an earplug microphone will be the same as the control attenuation at the end of the ear canal, i.e. at the surface of the tympanic membrane.
At issue in ANR earplug design is the need to control sound pressure at the eardrum over a wide frequency band. Prior research and design has focused on controlling the pressure at the earplug (or error) microphone. Prior ANR earplug designs have failed to appreciate the critical differences in reducing sound pressure at the earplug microphone in the control circuit versus reducing sound pressure at the user's tympanic membrane.
A number of ANR earplug designs are described in “An Active Noise reduction Ear Plug with Digitally Driven Feedback Loop,” by K. Buck, V. Zimpter and P. Hamery, a paper presented at Inter-Noise 2002, the International Congress and Exposition on Noise control Engineering, Aug. 19-21, 2002 (herein, “Buck”). One such design used a walkman-type loud speaker. According to Buck, the closed-loop performance of that design is not impressive, largely due to the electroacoustic transfer function of the walkman-type loud speaker.
Buck also describes a piezoceramic actuator. A flat-plate type device exhibited an electroacoustic transfer function that was amenable to ANR applications. However, the pressure output of the flat-plate piezoceramic actuator was insufficient for ANR applications, particularly in a noisy environment. A tube-type piezoceramic actuator was also tested and claimed as possible for use inside the ear canal, without specification of any dimensions. Like the flat-plate design, the transfer function of the tube-type piezoceramic actuator was acceptable, but the prototype device was too inefficient in output sound power for commercial applications. Buck concludes that the main obstacle in designing ANR earplugs is related to the transducers [actuators]. Controlling the pressure at the eardrum location not addressed by Buck.
Another paper, “Electroacoustic Design of an Active Earplug,” by Phillipe Herzog, a paper presented at Inter-Noise 2002, the International Congress and Exposition on Noise control Engineering, Aug. 19-21, 2002 (herein, “Herzog”), also discusses the design of earplugs with ANR. Herzog also comments on the design constraints posed by current choices for actuators:                 The piezoelectric speaker would allow to use a simpler control filter, but still require expensive developments. An cheaper solution, requiring also a simple control filter, would be electret speaker, if a relatively low pressure is to be controlled. Conversely, the emergence efficient numerical control filters may allow us to use existing dynamic speakers. In any case, the maximum pressure inside the ear canal remains a critical criterion.        
Buck and Herzog both focus on the actuator as a main obstacle in the design of an effective earplug with ANR system. Both papers fail to address how to design an earplug so that the electronic ANR performance can be compared to, and tailored to match the performance at the tympanic membrane.
A publication by Thomas R. Harley, et al., titled “Digital Active Noise reduction Ear Plugs,” Air Force Research Laboratory Report AFRL-HE-WP-TR-2001-0042, points out that ANR earplugs tested in the ear canal simulator of a KEMAR mannequin exhibited discrepancies that showed “. . . cancellation at the earplug microphone did not guarantee cancellation at the (mannequin) eardrum.” However, human test subjects were not tested to determine if this discrepancy was also observed for human tympanic membranes. In addition, the paper does not offer any explanations for this discrepancy nor does it provide any solutions to overcome critical differences in performance between the electronic ANR system and ANR performance at the eardrum of a KEMAR mannequin or the tympanic membrane of a human.
In U.S. Pat. No. 4,985,925 issued to Langberg et al. (Langberg) presents the idea of a small electroacoustic actuator used as part of a feedback system to close the loop on a second electroacoustic transducer (microphone) to suppress the sound pressure levels observed by the microphone over a relatively narrow band of frequencies (20 Hz to 1 kHz). The speaker and microphone locations for the earplug embodiments identified in Langberg are defined simply as “in proximity of an ear canal.” Figure 2 of Langberg indicates that the speaker is just outside of the ear canal and the microphone is located just inside the ear canal entrance. Langberg discusses the phase of the electroacoustic transfer function as a fundamental obstacle to electronic control performance. However, there is no discussion of the impact of transducer placement on noise reduction performance at the eardrum, or the human perceived performance of the resulting control.
U.S. Pat. No. 5,305,387 issued to Sapiejewski teaches the use of a first electroacoustic transducer (actuator) arranged inside the concha of a user, with a second electroacoustic transducer (microphone) situated inside a front cabinet volume and adjacent to the actuator, and intercoupling of first and second transducers with feedback electronic circuitry to actively reduce the noise intensity inside the concha cavity.
U.S. Pat. No. 5,631,965 issued to Chang discusses the use of a microphone to receive an external acoustic sound signal to be electronically processed in the feedforward sense for creation of a sound signal that travels down a tube extending through the ear piece to the ear canal.
U.S. Pat. No. 5,740,258 issued to Goodwin Johansson also discusses a feedforward active noise suppressor that includes an input transducer used to generate an electrical signal in response to sound pressure waves entering the ear canal, then is processed to generate an inverse noise signal applied to the output transducer. No specifications or embodiments that correlate the output transducer measurements to the tympanic membrane sound pressures are provided or discussed.
What would be useful is an electronic earplug that monitors and replicates sound pressures at the tympanic membrane thereby offering electronic enhanced noise reduction performance at the tympanic membrane over a specific frequency bandwidth. Such an electronic earplug would also provide accurate monitoring; and high-bandwidth noise reduction performance at the tympanic membrane for the required conditions of a hearing protection application. The accurate resemblance, or mapping, of complex sound pressures at the earplug microphone to the complex sound pressures at the tympanic membrane would also permit calculations of noise exposure limits for humans in hazardous noise fields.